1. Field of the Invention
This is a method for correcting seismic travel times, gathered along a line of survey in a body of water such as the open ocean, to compensate for an irregular water bottom in the presence of spatial or temporal dynamic changes in the physical properties of the water mass.
2. Description of the Prior Art
Sea water has an acoustic velocity near the surface and within the mixed layer of around 1540 meters per second (m/s). Thereafter the acoustic velocity drops off to an average velocity through the water mass of about 1500 m/s. The acoustic velocity of the earth layers beneath the sea floor is on the order of 2000 to 3500 m/s depending upon the rock type.
Hereinafter in this application, the term "water velocity" will be used as a shorthand substitute for the expression "the velocity of an acoustic (sound) wave propagating through a body of water." The term "water velocity" does not mean the velocity of an elemental volume of water moving as part of an ocean current such as the Gulf Stream.
For good and sufficient reasons, marine seismic data are presented as time scale recordings along a line of survey in terms of two-way reflection travel times to sub-bottom earth layers vs lateral station separation. Variations in the thickness of an overlying layer between stations will introduce a false travel-time differential to the arrival times to layers below the overlying layer. So long as the velocity difference between layers is small, the false time anomaly will be small to insignificant. At sea, the water velocity is much less than the formation velocity. The effect of a rough water bottom is to distort the reflection travel times to the respective sub-bottom earth layers such that they approximately mirror a profile of the sea floor. One way to correct the reflection time data is by use of a layer replacement technique. The method is a type of static correction wherein the objective it to determine the reflection arrival times which would have been observed on a flat plane with no intervening low velocity material, i.e., water, present. Application of that method requires accurate knowledge of the water depth or thickness at each station which, in turn, requires an accurate knowledge of the water velocity.
In the past, it has been customary to compute the water thickness at each seismic station along the line of survey with a fathometer using a built-in assumed constant water velocity such as 1500 m/s or perhaps 5000 feet per second, depending upon the desired units of measurement. All subsequent seismic-data reduction computations were referred to that depth measurement.
The fallacy of the time-honored correction method at sea is the fact that the water velocity is not at all constant on an area-wide basis. It varies considerably not only laterally within a region but also time-wise due to shifting currents and eddies. For example, in the Gulf of Mexico, a velocity variation between summer and fall of 40 m/s at a depth of 500 meters was measured. As a consequence, intersecting seismic lines of survey shot at different times of year have experienced serious data misties amounting to as much as 12 to 20 milliseconds (ms) when corrections for a variable water depth were made using the aforementioned constant assumed average water velocity.
The water velocity is a function of salinity, density, pressure and temperature. It is known that the ocean water is distinctly stratified. Historically, the water velocity is calculated from information gleaned from bathythermograph and Nansen bottle casts which provide the data to solve an equation for the water velocity as a function of depth such as that shown in the Encyclopedic Dictionary of Exploration Geophysics, by R. E. Sheriff, page 270. The Bissett-Berman SVDT system has also been used to get a continuous velocity-temperature profile of the water mass. Oceanographic stations are generally several tens of miles apart. In making oceanographic casts, the instruments are secured to wire lines that may be many thousands of feet long. The oceanographic ship is obliged to linger one or more days on-station to make a single cast. The popularly-used assumed constant water velocity is simply a convenient round number derived from a world-wide average of data from sample points that are widely separated in space and time.
Heretofore, seismic exploration contractors, working close to shore, have been guilty of ignoring variations in the water velocity as a function of depth, location and time of year. So long as the water remained shallow and the water bottom remained relatively flat, no harm was done. As exploration moves into deeper water, on the order of thousands of feet, the matter of a laterally- and temporally-varying water velocity presents a serious problem. Certainly, a commercial geophysical exploration crew, which is expected to occupy hundreds of stations per day, cannot afford to make time-consuming oceanographic casts at each station.
Given a flat ocean bottom, the RMS velocity of the water mass can be calculated from the seismic data itself, that is from the ocean bottom reflections, through the medium of an X.sup.2 -T.sup.2 analysis such as described at page 282 of Sheriff (op cit). However, that ideal situation of a flat bottom is the exception rather than the rule. In deep water, the sea floor is usually far too irregular to be used for a velocity study of the overlying water mass.
I have discovered that, with careful attention given to instrumentation and noise control in the field along with innovative data processing, I can observe and record reflections from discontinuities within the water mass itself. From those reflections, I can measure the acoustic properties of the water mass at each seismic station concurrently with the gathering of conventional seismic reflection data from the earth layers beneath the water bottom. I can do that by using weak data recorded during the time window between the shot instant and the arrival of the water-bottom reflection that others, skilled in the art, failed to recognize as being valid information and which they previously threw away as being interfering noise.